Advances in cell-replacement therapy for Type I diabetes mellitus and a shortage of transplantable islets of Langerhans have focused interest on developing sources of insulin-producing cells, or β cells, appropriate for engraftment. One approach is the generation of functional β cells from pluripotent stem cells, such as, for example, embryonic stem cells.
In vertebrate embryonic development, a pluripotent cell gives rise to a group of cells comprising three germ layers (ectoderm, mesoderm, and endoderm) in a process known as gastrulation. Tissues such as, for example, thyroid, thymus, pancreas, gut, and liver, will develop from the endoderm, via an intermediate stage. The intermediate stage in this process is the formation of definitive endoderm. Definitive endoderm cells express a number of markers, such as, HNF-3 beta, GATA4, MIXL1, CXCR4 and SOX17.
Formation of the pancreas arises from the differentiation of definitive endoderm into pancreatic endoderm. Cells of the pancreatic endoderm express the pancreatic-duodenal homeobox gene, PDX1. In the absence of PDX1, the pancreas fails to develop beyond the formation of ventral and dorsal buds. Thus, PDX1 expression marks a critical step in pancreatic organogenesis. The mature pancreas contains, among other cell types, exocrine tissue and endocrine tissue. Exocrine and endocrine tissues arise from the differentiation of pancreatic endoderm.
Cells bearing the features of islet cells have reportedly been derived from embryonic cells of the mouse. For example, Lumelsky et al. (Science 292:1389, 2001) report differentiation of mouse embryonic stem cells to insulin-secreting structures similar to pancreatic islets. Soria et al. (Diabetes 49:157, 2000) report that insulin-secreting cells derived from mouse embryonic stem cells normalize glycemia in streptozotocin-induced diabetic mice.
In one example, Hori et al. (PNAS 99: 16105, 2002) disclose that treatment of mouse embryonic stem cells with inhibitors of phosphoinositide 3-kinase (LY294002) produced cells that resembled β cells.
In another example, Blyszczuk et al. (PNAS 100:998, 2003) reports the generation of insulin-producing cells from mouse embryonic stem cells constitutively expressing Pax4.
Micallef et al. reports that retinoic acid can regulate the commitment of embryonic stem cells to form Pdx1 positive pancreatic endoderm. Retinoic acid is most effective at inducing Pdx1 expression when added to cultures at day 4 of embryonic stem cell differentiation during a period corresponding to the end of gastrulation in the embryo (Diabetes 54:301, 2005).
Miyazaki et al. reports a mouse embryonic stem cell line over-expressing Pdx1. Their results show that exogenous Pdx1 expression clearly enhanced the expression of insulin, somatostatin, glucokinase, neurogenin3, P48, Pax6, and HNF6 genes in the resulting differentiated cells (Diabetes 53: 1030, 2004).
Skoudy et al. reports that activin A (a member of the TGF-β superfamily) upregulates the expression of exocrine pancreatic genes (p48 and amylase) and endocrine genes (Pdx1, insulin, and glucagon) in mouse embryonic stem cells. The maximal effect was observed using 1 nM activin A. They also observed that the expression level of insulin and Pdx1 mRNA was not affected by retinoic acid; however, 3 nM FGF7 treatment resulted in an increased level of the transcript for Pdx1 (Biochem. J. 379: 749, 2004).
Shiraki et al. studied the effects of growth factors that specifically enhance differentiation of embryonic stem cells into Pdx1 positive cells. They observed that TGF-β2 reproducibly yielded a higher proportion of Pdx1 positive cells (Genes Cells. 2005 June; 10(6): 503-16.).
Gordon et al. demonstrated the induction of brachyury+/HNF-3beta+ endoderm cells from mouse embryonic stem cells in the absence of serum and in the presence of activin along with an inhibitor of Wnt signaling (US 2006/0003446A1).
Gordon et al. (PNAS, Vol 103, page 16806, 2006) states “Wnt and TGF-beta/nodal/activin signaling simultaneously were required for the generation of the anterior primitive streak”.
However, the mouse model of embryonic stem cell development may not exactly mimic the developmental program in higher mammals, such as, for example, humans.
Thomson et al. isolated embryonic stem cells from human blastocysts (Science 282:114, 1998). Concurrently, Gearhart and coworkers derived human embryonic germ (hEG) cell lines from fetal gonadal tissue (Shamblott et al., Proc. Natl. Acad. Sci. USA 95:13726, 1998). Unlike mouse embryonic stem cells, which can be prevented from differentiating simply by culturing with Leukemia Inhibitory Factor (LIF), human embryonic stem cells must be maintained under very special conditions (U.S. Pat. No. 6,200,806; WO 99/20741; WO 01/51616).
D'Amour et al. describes the production of enriched cultures of human embryonic stem cell-derived definitive endoderm in the presence of a high concentration of activin and low serum (Nature Biotechnology 2005). Transplanting these cells under the kidney capsule of mice resulted in differentiation into more mature cells with characteristics of some endodermal organs. Human embryonic stem cell-derived definitive endoderm cells can be further differentiated into Pdx1 positive cells after addition of FGF-10 (US 2005/0266554A1).
D'Amour et al. (Nature Biotechnology-24, 1392-1401 (2006)) states: “We have developed a differentiation process that converts human embryonic stem (hES) cells to endocrine cells capable of synthesizing the pancreatic hormones insulin, glucagon, somatostatin, pancreatic polypeptide and ghrelin. This process mimics in vivo pancreatic organogenesis by directing cells through stages resembling definitive endoderm, gut-tube endoderm, pancreatic endoderm and endocrine precursor en route to cells that express endocrine hormones”.
In another example, Fisk et al. reports a system for producing pancreatic islet cells from human embryonic stem cells (US2006/0040387A1). In this case, the differentiation pathway was divided into three stages. Human embryonic stem cells were first differentiated to endoderm using a combination of sodium butyrate and activin A. The cells were then cultured with TGF-β antagonists such as Noggin in combination with EGF or betacellulin to generate Pdx1 positive cells. The terminal differentiation was induced by nicotinamide.
In one example, Benvenistry et al. states: “We conclude that over-expression of Pdx1 enhanced expression of pancreatic enriched genes, induction of insulin expression may require additional signals that are only present in vivo” (Benvenistry et al, Stem Cells 2006; 24:1923-1930).
Cyclins have been implicated in beta cell function. For example, Lilja et al report that Cdk5 is present in the insulin-secreting pancreatic β-cell (J. Biol. Chem., Vol. 276, Issue 36, 34199-34205, Sep. 7, 2001). Lilja et al states “Cdk5 is present in β-cells and acts as a positive regulator of insulin exocytosis.”
In another example, Marzo et al states “Cdk4 knockin mice have significantly increased beta cell mass and are physiologically functional, indicating that Cdk4 is a potential target for pancreatic beta cell mass regeneration in Type 1 diabetes” (Diabetalogia, Vol. 47, Number 4, 686-694, Apr. 1, 2004.)
In another example, Ubeda et al report that inhibition of cyclin-dependent kinase 5 activity protects pancreatic beta cells from glucotoxicity (J. Biol. Chem., Vol. 281, Issue 39, 28858-28864, Sep. 29, 2006).
In another example, Wei et al report Cdk5-dependent regulation of glucose-stimulated insulin secretion (Nature Medicine 11, 1104-1108 (1 Oct. 2005.)
In another example, Vanderford et al state “MafA is a basic leucine zipper transcription factor expressed within the beta cells of the pancreas and is required to maintain normal glucose homeostasis as it is involved in various aspects of beta cell biology. MafA protein levels are known to increase in response to high glucose through mechanisms that have yet to be fully characterized. We investigated whether discrete intracellular signaling events control mafA expression. We found that the general kinase inhibitor staurosporine induces mafA expression without altering the stability of the protein. Inhibition of the MAP-kinase JNK mimics the effects of staurosporine on the expression of mafA. Calmodulin kinase and calcium signaling are also important in stimulating mafA expression by high glucose. However, staurosporine, JNK, and calmodulin kinase have different effects on the induction of insulin expression. This data reveals that MafA levels are tightly controlled by the coordinated action of multiple kinase pathways.” (Archives of Biochemistry and Biophysics (2008), doi: 10.1016/j.abb.2008.10.001).
Therefore, there still remains a significant need to develop methods for differentiating pluripotent stem cells into pancreatic endocrine cells, pancreatic hormone expressing cells, or pancreatic hormone secreting cells. The present invention provides methods to increase the expression of MAFA in cells expressing markers characteristic of the pancreatic endocrine lineage.